Showing papers by "Chris D. Thomas published in 1990"

What Do Real Population Dynamics Tell Us About Minimum Viable Population Sizes

Abstract: There is no single "magic" population size that guarantees the persistence of animal populations (e.g., Shaffer 1981; Gilpin & Soule 1986; Soule and Simberloff 1986; Lande 1988; Simberloff 1988). This is partly because extinction is probabilistic and partly because each minimum viable population (MVP) must be estimated separately, after considering characteristics of the population and environment under scrutiny. MVP estimates are then used to design and manage reserves for focal species. This approach is exemplified by studies reported in the Special Section on Population Viability Analysis in the March 1990 issue of Conservation Biology. The papers by Murphy et al. (1990) and Menges (1990) emphasize aspects of the dynamic nature of the spatial distributions of populations, and they stress the need to conserve what could be termed Minimum Dynamic Areas, often in reserve networks. Yet in the few remote and relatively unmodified regions of the world where reserve design is still possible on a large scale (e.g., parts of Amazonia), many focal species (e.g., large forest eagles) are too poorly known to calculate their MVPs and Minimum Dynamic Areas. Land use decisions will be made before the detailed information necessary to calculate specific MVPs has been obtained and incorporated into appropriate models. It might, however, be feasible to estimate population densities of representative focal species, and measure their home ranges (Thiollay 1989). This less detailed information could then be used to make provisional conservation recommendations, sometimes decades before more accurate MVP and Minimum Dynamic Area estimates could become available. To do this requires MVP guidelines, if not

The prevalence of introduced Vespula vulgaris wasps in a New Zealand beech forest community.

Abstract: In honeydew beech forest in the South Island of New Zealand, introduced Vespula vulgaris wasps are now very abundant. Approximated biomass estimates indicate that Vespula (mostly V. vulgaris) biomass (mean estimate at peak = 3761 g ha, averaged over the year = 1097 g ha) is as great as, or greater than combined biomasses of birds (best estimate = 206 g ha), rodents (up to 914 g ha in some years, but usually much lower) and stoats (up to 30 g ha). Relative V. vulgaris biomass is also estimated to be two orders of magnitude greater than native wasp biomass during the peak V. vulgaris season in one beech forest. Mean density of Vespula workers at the peak of the season was estimated to be 10,000 workers ha, a greater density than the densities attained by other large wasp species when they have been used (with some success) at 'enhanced densities' as biological control agents overseas. The biological impacts of Vespula, and particularly V. vulgaris, in honeydew beech forest are likely to be great, but most of these impacts have not been documented.

Herbivore diets, herbivore colonization, and the escape hypothesis

Abstract: The escape hypothesis predicts that isolated individuals of a given plant species will escape their specialized herbivores and so survive better than individuals growing in clumps. This phenomenon could promote plant coexistence. I examined as- sumptions underlying the hypothesis by considering insect herbivores associated with neo- tropical Passiflora vines. I found that the hypothesis was probably of only limited value because plants were attacked by a range of herbivore species, most of which were not really specialized. Furthermore, some oligophagous herbivore species colonized isolated P. pittieri more readily than they colonized plants that were near to other P. pittieri plants: isolated plants did not escape herbivory. Because of the activities of herbivores that feed on > 1 plant species, -2 plant species that share herbivores may not show independent patterns of herbivory or independent local distributions.

Diet divergence in two sympatric congeneric butterflies: community or species level phenomenon?

Abstract: Two species ofEuphydryas butterflies were studied in California, USA, and showed considerable diet overlap at the species level. They utilize many of the same plant genera for oviposition. However,E. editha is less likely to use woody perennials than isE. chalcedona. Both butterfly species are known to specialize on different host plants in different populations, so species level divergence may not be a good predictor of community level divergence. Within five communities,E. editha andE. chalcedona showed no dietary overlap. A major component of the niche ofE. editha in one community was occupied byE. chalcedona in a second community, even though both butterfly species occupied both communities. These resource use patterns indicate that community level interactions may affect diet divergence. The degree to which divergence within communities is greater (or less) than expected from a species level comparison may be used to provide a measure of community organization. Equations are given in the Appendix for calculating overlap probabilities from presence/absence types of data; in this study, presence is oviposition on a particular plant species, absence is no oviposition on that plant species. Given the various assumptions of the model,E. editha andE. chalcedona show significant community level components of their dietary divergence in the areas studied. However, in some other communitiesE. editha andE. chalcedona do share host plant species. Therefore, we could not demonstrate community level divergence conclusively, nor has this been demonstrated for any other pair of insect herbivore species. We do not know whether this is because the phenomenon is truly rare or just very hard to demonstrate.

Diet divergence in two butterflies: community sympatric congeneric or species level phenomenon?

Abstract: Summary Two species of Euphydryas butterflies were studied in California, USA, and showed considerable diet overlap at the species level. They utilize many of the same plant genera for oviposition. However, E. editha is less likely to use woody perennials than is E. chalcedona. Both butterfly species are known to specialize on different host plants in different populations, so species level divergence may not be a good predictor of community level divergence. Within five communities, E. editha and E. chalcedona showed no dietary overlap. A major component of the niche of E. editha in one community was occupied by E. chalcedona in a second community, even though both butterfly species occupied both communities. These resource use patterns indicate that community level interactions may affect diet divergence. The degree to which divergence within communities is greater (or less) than expected from a species level comparison may be used to provide a measure of community organization. Equations are given in the Appendix for calculating overlap probabilities from presence/absence types of data; in this study, presence is oviposition on a particular plant species, absence is no oviposition on that plant species. Given the various assumptions of the model, E. editha and E. chalcedona show significant community level components of their dietary divergence in the areas studied. However, in some other communities E. editha and E. chalcedona do share host plant species. Therefore, we could not demonstrate community level divergence conclusively, nor has this been demonstrated for any other pair of insect herbivore species. We do not know whether this is because the phenomenon is truly rare or just very hard to demonstrate.